A known Raman spectroscopy system is described in U.S. Pat. No. 5,442,438, which is incorporated herein by reference. FIG. 1 of the accompanying drawings shows a commercially-available embodiment of such a system.
In FIG. 1, light 13 from a laser source 10 is reflected by mirrors 12,14 and by a notch or edge filter 16 which acts as a dichroic beamsplitter. This directs it along an optical path 15 into a microscope 18, where it is deflected by a mirror 20 through an objective lens 22 and focused on a sample 24. Raman scattering takes place at the sample, producing Raman-shifted light at different wavenumbers from the incident laser line. The Raman-shifted light is collected by the objective lens 22 and passed back along the optical path 15 via the mirror 20 to the filter 16.
Whereas the filter 16 reflects light of the laser wavelength, it transmits the Raman-shifted wavenumbers. While doing so, it rejects the much more intense laser line. Further rejection of the laser line takes place in a second, identical filter 26. The Raman-shifted light then passes through a Raman analyser 28, which as described in U.S. Pat. No. 5,442,438 may comprise a diffraction grating, or filters which accept specific Raman lines of interest. The resulting light is then passed to a detector 30. This may for example comprise a charge-coupled device (CCD), across which a Raman spectrum may be dispersed by a diffraction grating. Or a filter may pass a two-dimensional image of the sample to the CCD, in light of a selected Raman wavenumber.
The notch or edge filters 16,26 may be holographic filters, as described in U.S. Pat. No. 5,442,438. Or they may be thin film multi-layer dielectric filters, such as for example the hard oxide filters supplied by Semrock Inc, Rochester, N.Y., USA under the trademark RazorEdge. Such filters are described in U.S. Pat. No. 7,068,430, incorporated herein by reference.
The filter 16 is necessarily placed at an angle to the optical path, in order to inject the light from the light source 10 towards the sample 24. However, in order to provide a sharp cut-off between the rejection of the laser line and the acceptance of Raman-scattered light at wavenumbers close to the laser line, U.S. Pat. No. 5,442,438 describes that this angle should be a low angle of incidence, such as 10°. The second filter 26 is similarly placed at the same low angle of incidence, to provide matching performance. In practice, angles of between 7.5° and 13° are used, but other angles are also possible.
Even at such low angles of incidence, however, polarisation effects reduce the sharpness of the cut-off. The larger the angle of incidence, the greater the problem. Specifically, the cut-off for p-polarised light is different from that for s-polarised light, by an amount which depends on the angle of incidence. It follows that the transmission characteristic of the filter shows a step or shelf in the cut-off edge at around 50% transmission for randomly polarised light. This results in polarisation artefacts in the resulting spectra measured by the device.